1. Field of the Invention
The present invention relates, generally, to optical analyzing instruments and, more particularly, to optical analyzing instruments performing near infrared quantitative analysis.
2. Description of the Related Art
The percentage concentration of grain or milled grain constituents partially determines its economic or market value. Often, the value of the grain may depend upon its protein, oil and moisture concentration, or combination thereof. This may hold particularly true during the milling process of wheat where it is desirable to remove the outer portion of the kernel (sometimes referred to as "bran" or "flour ash") from the milled flour. Improperly milled flour may be very wasteful, time inefficient and, further, increase milling cost if not corrected within a reasonable time frame.
Traditionally, chromatographic methods or indirect separation chemical analysis were employed as the only accurate constituent analysis techniques. The individual constituents were chemically separated from the specimen and analyzed for percentage concentration. For example, the Kjeldahl technique is a traditional analytical laboratory technique used to determine the protein content. Techniques such as these, although highly accurate, are relatively time consuming and generally require skilled chemists to conduct most technical operations. Typically, at least four hours are necessary to perform this analysis. Accordingly, it could be at least four hours before improperly milled flour is even discovered.
In the mid-sixties, Karl H. Norris of the USDA, observed that the absorbtivity, as determined by transmissivity or reflectivity, of near-infrared (NIR) light energy incident on a specimen surface is proportional to its composition and constituent concentrations. That is, quantitative diffuse reflectance in the near-infrared region is a function of the percentage of an analyte's concentration. Therefore, because particular grain constituents actively respond to at least one particular NIR light energy wavelength, by applying correlation spectroscopy the constituent concentration may be determined.
Typically, to analyze a grain specimen using NIR correlation spectroscopy, the sample is irradiated with preselected filtered light energy as it passes from a source, generally a wide band wavelength quartz tungsten-halogen light source or a narrow band Near-Infrared Emitting Diodes (NIREDs), to irradiate the sample. Currently, two types of NIR quantitative analysis instruments are commercially available. One type measures the sample diffuse reflectance while the other measures the sample transmissivity. In either type, the reflectivity or transmissivity of the preselected wavelength light incident on the sample, as set forth above, is a function of its constituent concentration.
It is well known that the analyte concentration is functionally proportional to the spectroscopic response by dither individual wavelengths, by difference between pairs of wavelengths, and by trios, where the reflectance of the central wavelength is mathematically weighted in comparison to the contribution of reflectance at wavelengths incrementally spaced on either side. Typical of such an individual wavelength algorithm is set forth below: EQU %.sub.Const =a.sub.0+ a.sub.1 log(1/R.sub.1)+a.sub.2 log(1/R.sub.2)+ . . . +a.sub.n log(1/R.sub.n)
where a.sub.0 =calibration offset,
a.sub.1,2, . . . n =are calculated calibration coefficients, and PA1 R.sub.1,2, . . . n =(I.sub.n /I.sub.r), i.e., the intensity of the preselected reflected from a sample light as compared to the intensity of the preselected reflected light from a reference.
Moreover, simultaneous multicomponent analysis is possible through the solution of multiple equations, such as: EQU %.sub.Oil =a.sub.0 +a.sub.1 log(1/R.sub.0)+a.sub.2 log(1/R.sub.w)+a.sub.3 log(1/R.sub.p); EQU %.sub.Water =a.sub.4 +a.sub.5 log(1/R.sub.0)+a.sub.6 log(1/R.sub.w)+a.sub.7 log(1/R.sub.p); EQU %.sub.Protein =a.sub.7 +a.sub.8 log(1/R.sub.0)+a.sub.9 log(1/R.sub.w)+a.sub.10 log(1/R.sub.p),
for example. Through these selected correlation spectroscopy algorithms and corresponding calibration offsets and coefficients, the particular constituent concentrations can be predicted from the diffuse reflectance intensities measured at different wavelengths. This technique is set forth and better detailed in the article entitled "Near-Infrared Reflectance Analysis" by D.L. Wetzel, Analytical Chemistry, Vol. 55, p. 1165A (1983).
Typically, a quartz tungsten-halogen light source emitting a wide band of wavelengths is used to irradiate the specimen through discrete selected wavelength interference filters mounted to a rotatable turret. These turrets include a plurality of radially positioned filters, each of which is designed to pass light at a preselected wavelength which are known to be optically reactive in determining the percentage concentration of the measured constituents.
In a limited capacity, computational circuitry, coupled to stored sensor signals and precalculated coefficients, have permitted this analysis technique to be commercially feasible. The agricultural community has increasingly relied on quantitative optical analyzers to determine moisture, starch, protein and oil, for example, in grain products. Typical of these devices are the perpendicularly impinging light sources described in U.S. Pat. Nos. 3,828,173, 4,236,076, 4,286,327 and 4,404,642, and the tilting interference filter systems set forth in U.S. Pat. Nos. 3,861,788 and 4,082,464.
These above-mentioned systems have often proved effective in determining certain grain constituent concentrations in milled and non-milled grains, such as protein, starch and moisture. One problem, however, is that these techniques are difficult to apply when the variety of wheat flour changes. Thus, each variety has heretofore required time-consuming recalibration of the equipment.
Mineral ash of a flour sample is directly proportional to the amount of bran layer of a wheat kernel that is left in the flour after the milling process. Thus, "ash" concentration determination is an important indicator of the undesirable residual mineral content not removed during milling.
Although it is possible through NIR optical analysis to predict concentrations of flour "ash" in milled wheat grain, the traditional preselected NIR wavelengths in the range of 1445 nm to 2348 nm have not yielded consistent results, particularly as the wheat variety changes. Commercial NIR instrumentation for flour ash usually employ standard combinations of interference filters, such as 1680 nm in combination with 2336 nm. Through correlation spectroscopy, these optical analyzers have been sufficient when calibrated for a particular blend of wheat flour. However, these instruments have been unreliable for predicting "ash" when applied to a wide variety of wheat flour. To obtain acceptable accuracy, recalibration (i.e., new set of calibration offsets and/or coefficients) is often necessary for each separate wheat flour blend. Therefore, the prior art quantitative optical analyzers have been extremely sensitive to matrix changes when quantifying flour "ash".
Ultimately, this recalibration problem is too burdensome for commercial feasibility. The flour grade is continually changing during the milling process. By the time a proper set of calibration offsets and coefficients are determined, that particular blend of wheat flour may be finished being milled and a new, slightly differing blend of wheat flour introduced.
Moreover, typical NIR optical analysis for wheat flour "ash" have been too sensitive to particle size variations. Unless the particle size difference from sample to sample is minimized, the optical analysis could not be accurately relied upon, even when using the proper calibration coefficients. Unfortunately, different wheat flour blends often differ in particle size. Although uniform particle size in milled flour is desirable, in order for these analyzers to function properly and accurately, it is often not practical.
Other optical techniques are sometimes employed to determine the flour "ash" concentration. For example, "ash" concentration in flour is sometimes estimated by the flour coloration alone. The brownish color of bran actively responds to the visible light range between 540 nm to 560 nm. Incidentally, this corresponds to the green to yellow color cross-over in the visible light spectrum. The basic premise is that wheat bran concentration in wheat flour discolors the wheat flour so that visual changes in the color can be detected. Thus, by measuring the tone or "color" of the flour, the mineral "ash" concentration may be estimated through a "color" correlation algorithm, similar to the constituent correlation algorithm used in NIR spectroscopy.
Unfortunately, these color correlation measurements also result in unreliable predictions when employed in attempting to analyze changing wheat flour blends. Because of the slightly different wheat kernel colors, recalibration is often necessary for each blend if accurate predictions are desired. Moreover, any other impurities which discolor the wheat flour may be construed as "ash" impurities. Thus, the measurements may falsely represent the flour "ash" concentration and lead to incorrect estimations. These techniques are not practical for commercial usage where analytical accuracy is compromised for practicality.
Accordingly, it is an object of the present invention to provide an optical analyzing apparatus and method which more accurately determines the chemical constituent concentrations of a specimen.
It is another object of the present invention to provide an optical analyzing apparatus and method which more precisely and accurately determines the flour "ash" for a wider variety of milled wheat flour.
Still another object of the present invention is to provide an optical analyzing apparatus and method which is analytically applicable to a wider range of sample particle sizes.
Yet another object of the present invention to provide an optical analyzing apparatus and method which can be accurately and efficiently be employed during the milling process.
It is a further object of the present invention to provide an optical analyzing apparatus and method which is durable, compact, easy to maintain, has a minimum number of components, is easy to use by unskilled personnel, and is economical to manufacture.
The apparatus and method of the present invention has other objects and features of advantage which will be more readily apparent from the following description of the Best Mode of Carrying Out the Invention and the appended claims, when taken in conjunction with the accompanying drawing.